I was told these markings on the wall are from "sanding". I believe the walls are plaster. I wasn't sure if I should believe it since it's in a strange pattern. However, it might be plausible since this is the only room in the house where the baseboards have not been finished yet (just sanded and not re-finished).

black marking on wallBlack marking on wall

  • Very much doubt they are from sanding, unless they were painted over and you sanded the paint off. They seem to be where air leaks from downstairs and could be dust, but looks more like mold. Is downstairs on the damp side?
    – crip659
    Sep 16 '21 at 13:36
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    Did you sand the walls or only the baseboards/woodwork? Are the black stains able to be rubbed/smudged off with your fingers or scraped off with a tool (like a putty knife)? Are they powdery? Can you remove them with water/cleaner? I can't imagine a scenario where they're because of sanding...or that they're mold without seeing evidence of water damage at the baseboards, plaster, or floor...
    – gnicko
    Sep 16 '21 at 13:56
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    Guessing the dust collection bag was black and it rubbed the wall. At first look, thought it was from a hardwood flooring hammer.
    – Mazura
    Sep 16 '21 at 14:25
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    It looks like soot. Does it smell like soot? It looks like the paint has been scraped off the wood: sometimes people use a torch to make the old paint bubble up. Also, what is directly underneath the floor in the room below...a wood stove with a leaky stack? Sep 16 '21 at 14:35
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    Based on the arrows in the photograph and your "have to go back" comment, I presume this is a house you're considering purchasing, not one you already own. I'd suggest asking the seller how, exactly, those marks are the result of sanding. @Mazura's suggestion of the black dust bag rubbing on the wall makes the most sense (if sanding is the case), but leave it to them to tell you, don't give them a prompt.
    – FreeMan
    Sep 16 '21 at 15:31

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